1. Field of Invention
The present invention relates to a circuit for suppressing audio noise, and particularly to a circuit for suppressing audio noise when the power supply is turned on and turned off.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a sound system using a conventional circuit for suppressing audio noise. Referring to FIG. 1, the sound system 10 includes an audio signal generator 11, a preamplifier 12, a power amplifier 13, a speaker 14 and a circuit for suppressing audio noise 15. Wherein, the audio signal generator 11 is, for example, a microphone, CD/VCD/DVD player or an AM/FM tuner, all of which are used for generating an audio signal Audio 1. The preamplifier 12 is provided power by a voltage supply V_PAMP and used for receiving and amplifying the audio signal Audio1, wherein a capacitor C3 is used for smoothing a possible excessive voltage fluctuation import from the voltage supply V_PAMP. The power amplifier 13 is provided power by another voltage supply V_PWR and used for receiving an audio signal Audio2 and increasing the load drive capability of the audio signal Audio2, so that the amplified audio signal is able to drive a speaker 14, wherein a capacitor C5 serves as the capacitor C3 for stabilizing voltage.
As the voltage supply V_PAMP is turned on and turned off, the audio signal Audio1 would be mixed with unwanted noise signal and be amplified by the preamplifier 12, so that a thunderous pop noise will explode. Therefore, a circuit for suppressing audio noise 15 is equipped between the preamplifier 12 and the power amplifier 13 for detecting and suppressing the audio noise.
FIG. 2 is a waveform diagram of an audio signal Audio2 inputted into the power amplifier 13 in FIG. 1. Referring to FIGS. 1 and 2, after the voltage supply V_PAMP are turned on, the voltage needs to take a rise duration to rise to a stable state (for example, in the stable state, the voltage maintains at 12 volts (V)). Similarly, after the voltage supply V_PAMP are turned off, the voltage needs to take a fall duration to fall to zero.
During the early stage of the voltage supply V_PAMP being turned on and the voltage being rising, a forward bias of around 0.7V is required to turn on a diode D1 and consequently a forward bias between the emitter and the base of a transistor T1 are not available, so that no current flow between the emitter and the collector of the transistor T1. At this point, the voltage at the base of another transistor T2 is zero; therefore, the audio signal Audio2 mixed with noise is directly sent to the power amplifier 13, which makes the speaker 14 to produce a thunderous pop noise shown as block 21 in FIG. 2. Normally, only after the voltage of the voltage supply V_PAMP rises to around 2.5V, the circuit for suppressing audio noise 15 is able to suppress the noise.
Similarly, during the early stage of the voltage supply V_PAMP being turned off and the voltage being falling, the emitter voltage of the transistor T1 is less than the base voltage, therefore the noise can not be effectively suppressed and the speaker 14 produces a thunderous pop noise shown as block 22 in FIG. 2. Only after the voltage of the voltage supply V_PAMP falls to such extent that the emitter voltage of the transistor T1 is larger than the base voltage, the circuit for suppressing audio noise 15 is able to suppress the noise.